While much of the media focus on the effects of climate change has been on the Arctic, a review of peer-reviewed scientific literature done by researchers at Australia's James Cook University reveals that in the past 25 years there's been a expansion of the world's tropical zones and that human activity has contributed to it:The literature review shows that the areas which climatologists and meteorologists consider to be the tropics (which is defined differently than in geography) have expanded at minimum 300 kilometers (186 miles).

Future expansion of these zones is harder to predict, but based on what's now known the planet could see a further spread of the tropical conditions over the next 25 years of between 222-553 kilometers (138-338 miles).

Sub-Tropical Arid Zones Push Into New AreasThe implication of this is that sub-tropical arid zones which border the tropics are being pushed into areas which previously had a more temperate climate, with more pronounced differences in seasonal temperatures and precipitation.

As these areas expand they will begin pushing into more heavily populated areas, with potentially severe impacts on society.

Such areas include heavily populated regions of southern Australia, southern Africa, the southern Europe-Mediterranean-Middle East region, the south-western United States, northern Mexico, and southern South America – all of which are predicted to experience severe drying.

If the dry subtropics expand into these regions, the consequences could be devastating for water resources, natural ecosystems and agriculture, with potentially cascading environmental, social and health implications.

Tropical Diseases Spread Into New AreasFurthermore, the expansion of tropical diseases such as dengue fever could hit areas where the diseases was previously not endemic or where epidemic levels of dengue were previously not present. Researchers said the literature showed that these areas include the southern United States, China, northern Africa in the northern hemisphere, and parts of South America, southern Africa, and most of Australia in the southern hemisphere.

Anthropogenic Causes Pointed ToAs to the cause of all this, the literature is less certain, though the most recent research points to anthropogenic causes. Research published earlier this year in the journal Geophysical Research Letters shows,

...that the observed widening of the tropics can only be accurately replicated by an atmospheric general circulation model that includes direct radiative effects related to human greenhouse gas emissions and stratospheric ozone depletion. Simulation models which did not include anthropogenic radiative effects showed no significant change in the width of the tropics.